The invention relates generally to information carriers. Of particular interest to the invention are information or record carriers on which information may be stored or recorded by means of a beam of high-energy radiation.
Information carriers of the above type may be of plate-like configuration. The recording of information may be effected by relative movement between the radiation beam and the record plate and, in this manner, a mechanically, optically or electrically readable trace representing information or data may be produced on the plate. The radiation beam used may be a modulated beam such as a frequency-modulated beam.
For example, a known record plate comprises a plastic disc. Information is recorded on the disc using a concentrated laser beam and the information trace produced in the disc is in the form of depressions which have been burned into the disc by the laser beam. After the recording operation, the trace is transferred to a mold and the mold is then used to produce a series of information-carrying plates by compression. Subsequently, a metallic substance is vapor-deposited on the thus-produced plates at an oblique angle so that the holes or depressions pressed into the plates during the molding operation remain optically transparent whereas the material adjacent the depressions becomes optically opaque. The information contained in a trace produced in this manner, that is, a trace composed of optically transparent depressions, may be read out again using a fine light beam and a photocell. In this procedure, the traces responsible for the signals are generally not provided on the molds in real time but, rather, are provided on the molds with an extension in the signal sequence.
To amplify upon this somewhat, it is pointed out that the term "real time" as used in this connection is intended to denote that time with which video signals are conventionally taken up and also reproduced, although it should be borne in mind that the invention is not solely concerned with video signals. For instance, the real time for a complete image of a video signal amounts of 1/30 of a second according to the appropriate U.S. Standard.
The recording processes which have become known in recent years such as, for example, those associated with wireless or Philips information carriers, have been directed towards the goal of enabling the information carriers to be reproduced relatively inexpensively. Thus, a master is first made in the manner outlined above. The production of the master is not, however, carried out by the user but, rather, is effected in a central factory. During the production of the master, this is moved quite slowly and, accordingly, the signals are recorded with an extension in time. In other words, the signals are not recorded in real time but are recorded such that they are drawn out or extended in time. As a result, the traces responsible for the signals are not provided on the mold in real time but are extended in time.
Although a procedure such as briefly outlined above does permit information to be recorded and retrieved, it is often desirable for the trace responsible for the signals to be provided on the information carrier in real time and for the signals to be obtained in real time.
Thus, for the production of information traces in real time by burning away or vaporizing certain regions of an information carrier, the U.S. Pat. No. 3,181,170 has already proposed materials such as cadmium, anthracene or a suitable plastic substance for use in forming a vaporizable layer. However, the reflectivity of metals such as cadmium, and also aluminum, for instance, as well as the vaporizing temperatures of such metals, are quite high. On the other hand, the absorbing power of anthracene and plastic materials for radiation is quite low. Hence, in either case, the laser energy required for the production of information traces, which energy determines the price of the laser, is too high.
These problems have been overcome to some extent by the recording medium described in Federal Republic of Germany patent application No. 1,574,687, which corresponds to our U.S. Pat. No. 3,560,994. Here, it has been proposed to use a two-layer configuration for the purpose of obtaining a radiation-susceptible arrangement which enables signal-producing traces to be achieved by means of a modulated radiation beam. One of the layers may consist of bismuth and the other layer may consist of selenium and the arrangement may be produced by first forming one layer from one of the elements and then forming the other layer from the other of the elements. This arrangement is, as such, suitable for the production of the signal patterns of video signals in real time.
It has, however, been found that the characteristics of this arrangement are not entirely satisfactory and that, with the even more stringent requirements being imposed on the density of the recorded information, further improvements in the characteristics of the recording layer or layers are desirable.
Thus, since the information produced by frequency-modulated recording is represented by the precise positions and lengths of the "burned in" holes or depressions, that is, the depressions formed by burning away certain regions of a layer, particularly stringent requirements are imposed on the reproducibility of these dimensions, i.e., the positions and lengths of the depressions. The variations may, at most, be of the order of 100 angstroms. This requires a very homogeneous material for the recording layer.
The desired homogeneity may be obtained with amorphous layers. However it is not sufficient merely to use an amorphous material for the layer. Thus, in addition to being amorphous, the material should possess characteristics which enable as low a recording energy as possible to be achieved.
Non-grainy organic layers are known. Such layers are suitable for recording purposes when a change in transparency can be achieved by irradiation and the recording of information can be accomplished in this manner. An example of a layer which is capable of recording information by a change in transparency is one produced by the Ozalid process. For layers of this type, the thickness of the layer is not particularly critical and even layer thicknesses as great as the order of 1 micron or so may be satisfactorily used.
However, if the recording procedure involves the removal of material from the recording layer by means of radiation, i.e., vaporization of the material of the recording layer, then an additional condition is imposed on the layer and, in particular, a condition is imposed on the thickness of the layer. Thus, in such a case, the thickness of the recording layer should amount to only a small fraction of the wavelength of the radiation used for the recording of the information on the layer. This requirement may be fulfilled with metallic layers which are already sufficiently opaque at thicknesses of a few hundred angstroms.
To date, however, no suitable amorphous recording layers of such thicknesses have been achieved.